Most automobiles have engines and transmissions. The engine and transmission is used to turn a driveshaft. The driveshaft is connected to a differential. The differential transfers the rotational energy of the driveshaft to the axles and wheels of the automobile.
Generally, when an automobile is driving in a straight line, the wheels on the left side of the vehicle and the wheels on the right side of the vehicle rotate at the same speed. However, when the automobile makes a turn, the wheels on the outside of the turn must travel farther than the wheels on the inside of the turn. Consequently, the wheels on the outside of the turn must rotate at a slightly faster rate than the inside wheels during the turn. The use of differentials enables opposing axles on opposite sides of the vehicle to rotate at different speeds. As such, the wheels of the vehicle can each rotate at the proper speed to accommodate a turn.
Many conventional differentials are designed based on operation when the wheels of the automobile are encountering good road conditions. In real life, this is not always the case. Roads are often covered in snow, ice, dirt, gravel, mud and the like that can make a wheel slip or skid during a turn. In such situations, differentials may misinterpret the conditions and allow too much power to be applied to the slipping wheel. This can adversely affect the safety of the vehicle.
In attempts to limit such traction problems, some vehicle manufacturers use limited-slip differentials. A limited-slip differential is a type of automotive differential gear arrangement that allows for some difference in angular velocity of the output shafts, but imposes mechanical boundaries on the disparity. In an automobile, such limited-slip differentials are sometimes used in place of a standard differential, where they convey certain dynamic advantages, at the expense of greater complexity. See, for example, U.S. Pat. No. 7,357,748 to Kelly, entitled Limited Slip Differential.
Prior art electronic limited slip differentials have certain innate problems. Being limited slip differentials, such prior art differentials do allow for some slip of the axles and wheels, albeit that the slip is limited in range. Under certain circumstances, even limited slip can detract from the safety of a vehicle. Torque transfer is another common problem associated with limited slip differentials. Limited slip differentials endeavor to redirect torque to the axle with the most traction. However, some torque is always lost to the axle of the slipping wheel. Furthermore, differentials are traditionally mechanical devices. The addition of electronics to differentials has increased their complexity and corresponding cost of manufacture. The use of electronic controls within the differential makes the differential expensive to build and difficult to repair.
A need therefore exists for an improved differential system that precisely controls a differential rotational rate between different axle shafts while allowing for no significant slip. A need also exists for a differential that has the precision of electronic controls, yet is simple and inexpensive to manufacture.